Pharmacokinetics of ifosfamide and its enantiomers following a single 1 h intravenous infusion of the racemate in patients with small cell lung carcinoma

Stereoselectivity in drug metabolism

Many chiral drugs are used as their racemic mixtures in clinical practice. Two enantiomers of a chiral drug generally differ in pharmacodynamic and/or pharmacokinetic properties as a consequence of the stereoselective interaction with optically active biological macromolecules. Thus, a stereospecific assay to discriminate between enantiomers is required in order to relate plasma concentrations to pharmacological effect of a chiral drug. Stereoselective metabolism of drugs is most commonly the major contributing factor to stereoselectivity in pharmacokinetics. Metabolizing enzymes often display a preference for one enantiomer of a chiral drug over the other, resulting in enantioselectivity. The structural characteristics of enzymes dictate the enantiomeric discrimination associated with the metabolism of chiral drugs. The stereoselectivity can, therefore, be viewed as the physical property characteristic that phenotypes the enzyme. This review provides a comprehensive appraisal of stereochemical aspects of drug metabolism (i.e., enantioselective metabolism and first-pass effect, enzyme-selective inhibition or induction and drug interaction, species differences and polymorphic metabolism).

Systemic anticancer therapy in gynecological cancer patients with renal dysfunction

Chronic kidney disease is a common occurrence in patients with gynecological cancer. Systemic anticancer treatment in such patients is a challenge for clinicians because of altered drug pharmacokinetics. For those drugs that are excreted mainly by the kidneys, decreased renal function may lead to increased systemic exposure and increased toxicity. Dose adjustment based on pharmacokinetic changes is required in this situation to avoid life-threatening toxicity. In this review, we summarize the nephrotoxicity and pharmacokinetic data of agents commonly used in systemic anticancer treatment of gynecological cancers and dose adjustment guidelines in the presence of impaired renal function. We review 17 medications that need dose adjustment (cisplatin, carboplatin, doxorubicin, epirubicin, cyclophosphamide, ifosfamide, topotecan, irinotecan, etoposide, capecitabine, bleomycin, methotrexate, actinomycin D, granulocyte-macrophage colony-stimulating factor, metoclopramide, cimetidine, and diphenhydramine) as well as 27 drugs that do not (paclitaxel, docetaxel, pegylated liposomal doxorubicin, gemcitabine, oxaliplatin, fluorouracil, vincristine, letrozole, anastrozole, tamoxifen, leuprorelin, megestrol, gefitinib, erlotinib, trastuzumab, leucovorin, granulocyte colony-stimulating factor, erythropoietin, ondansetron, granisetron, palonosetron, tropisetron, dolasetron, aprepitant, dexamethasone, lorazepam, and diazepam). We also review the formulae commonly used to estimate creatinine clearance, including Cockcroft-Gault, Chatelut, Jelliffe, Wright, and the Modification of Diet in Renal Disease study formulae.

Metabolism and transport of oxazaphosphorines and the clinical implications

The oxazaphosphorines including cyclophosphamide (CPA), ifosfamide (IFO), and trofosfamide represent an important group of therapeutic agents due to their substantial antitumor and immuno-modulating activity. CPA is widely used as an anticancer drug, an immunosuppressant, and for the mobilization of hematopoetic progenitor cells from the bone marrow into peripheral blood prior to bone marrow transplantation for aplastic anemia, leukemia, and other malignancies. New oxazaphosphorines derivatives have been developed in an attempt to improve selectivity and response with reduced toxicity. These derivatives include mafosfamide (NSC 345842), glufosfamide (D19575, beta-D-glucosylisophosphoramide mustard), NSC 612567 (aldophosphamide perhydrothiazine), and NSC 613060 (aldophosphamide thiazolidine). This review highlights the metabolism and transport of these oxazaphosphorines (mainly CPA and IFO, as these two oxazaphosphorine drugs are the most widely used alkylating agents) and the clinical implications. Both CPA and IFO are prodrugs that require activation by hepatic cytochrome P450 (CYP)-catalyzed 4-hydroxylation, yielding cytotoxic nitrogen mustards capable of reacting with DNA molecules to form crosslinks and lead to cell apoptosis and/or necrosis. Such prodrug activation can be enhanced within tumor cells by the CYP-based gene directed-enzyme prodrug therapy (GDEPT) approach. However, those newly synthesized oxazaphosphorine derivatives such as glufosfamide, NSC 612567 and NSC 613060, do not need hepatic activation. They are activated through other enzymatic and/or non-enzymatic pathways. For example, both NSC 612567 and NSC 613060 can be activated by plain phosphodiesterase (PDEs) in plasma and other tissues or by the high-affinity nuclear 3'-5' exonucleases associated with DNA polymerases, such as DNA polymerases and epsilon. The alternative CYP-catalyzed inactivation pathway by N-dechloroethylation generates the neurotoxic and nephrotoxic byproduct chloroacetaldehyde (CAA). Various aldehyde dehydrogenases (ALDHs) and glutathione S-transferases (GSTs) are involved in the detoxification of oxazaphosphorine metabolites. The metabolism of oxazaphosphorines is auto-inducible, with the activation of the orphan nuclear receptor pregnane X receptor (PXR) being the major mechanism. Oxazaphosphorine metabolism is affected by a number of factors associated with the drugs (e.g., dosage, route of administration, chirality, and drug combination) and patients (e.g., age, gender, renal and hepatic function). Several drug transporters, such as breast cancer resistance protein (BCRP), multidrug resistance associated proteins (MRP1, MRP2, and MRP4) are involved in the active uptake and efflux of parental oxazaphosphorines, their cytotoxic mustards and conjugates in hepatocytes and tumor cells. Oxazaphosphorine metabolism and transport have a major impact on pharmacokinetic variability, pharmacokinetic-pharmacodynamic relationship, toxicity, resistance, and drug interactions since the drug-metabolizing enzymes and drug transporters involved are key determinants of the pharmacokinetics and pharmacodynamics of oxazaphosphorines. A better understanding of the factors that affect the metabolism and transport of oxazaphosphorines is important for their optional use in cancer chemotherapy.

Enantioselective metabolism and cytotoxicity of R-ifosfamide and S-ifosfamide by tumor cell-expressed cytochromes P450

The anticancer prodrug ifosfamide (IFA) contains a chiral phosphorous atom and is administered in the clinic as a racemic mixture of R-IFA and S-IFA. Hepatic cytochrome P450 (P450) enzymes exhibit enantioselective preferences in the metabolism of R-IFA and S-IFA; however, the impact of this selectivity on P450-dependent anticancer activity is not known. Presently, the metabolism and cytotoxicity of R-IFA and S-IFA were determined in 9L gliosarcoma and Chinese hamster ovary tumor cells expressing an IFA-activating P450 enzyme and by in vitro steady-state kinetic analysis using cDNA-expressed P450 enzymes. Tumor cells expressing P450 enzyme CYP3A4 were the most sensitive to R-IFA cytotoxicity, whereas tumor cells expressing CYP2B1 or CYP2B6 were most sensitive to cyclophosphamide (CPA), an isomer of IFA. Correspondingly, CYP3A4-expressing cells and cDNA-expressed CYP3A4 metabolized R-IFA to yield the active, 4-hydroxylated metabolite at a 2- to 3-fold higher rate than they metabolized S-IFA or CPA. CYP2B cells and cDNA-expressed CYP2B enzymes metabolized CPA almost exclusively by 4-hydroxylation, whereas R-IFA and S-IFA were substantially converted to inactive, N-dechloroethylated metabolites. Further investigation revealed that CYP3A1, a rat enzyme, exhibited superior kinetic properties compared with the human enzyme CYP3A4, with R-IFA and S-IFA both metabolized with high catalytic efficiency by 4-hydroxylation and with a K(m) value of 200 microM, approximately 5-fold lower than CYP3A4. Based on these kinetic parameters and metabolic profiles, R-IFA is expected to exert greater anticancer activity than S-IFA or CPA against tumors that express CYP3A enzymes, whereas tumors expressing CYP2B enzymes may be more sensitive to CPA treatment.

Influence of age upon Ifosfamide-induced nephrotoxicity

BACKGROUND

Ifosfamide-induced nephrotoxicity is well recognized in children, although it has also been reported in adults. Whether ifosfamide nephrotoxicity is more common in children than in adults is not known.

PROCEDURE

Medical records of adults and children diagnosed with sarcoma whom received ifosfamide with a cumulative dose >20 g/m(2) were evaluated. Twenty-five children (</=18-years of age) and 28 adults were identified.

RESULTS

National Cancer Institute Common Toxicity Criteria grade 3-4 ifosfamide-induced nephrotoxicity was present in 24 and 17% of children and adults, respectively (P = 0.58). Cumulative ifosfamide doses were similar between the two populations, with the median (range) of 70.2 g/m(2) (22.4-72) for children and 59 g/m(2) (20.8-146) for adults (P = 0.25). Logistic regression analysis indicated that neither age or cumulative ifosfamide dose were associated with grade 3-4 ifosfamide-induced nephrotoxicity (P = 0.36).

CONCLUSIONS

Children and adults receiving >20 g/m(2) of ifosfamide have similar susceptibility to ifosfamide-induced nephrotoxicity. Factors other than age and cumulative dose should be considered for understanding the inter-individual variation in nephrotoxicity.

Clinical pharmacokinetics and pharmacodynamics of ifosfamide and its metabolites

This review discusses several issues in the clinical pharmacology of the antitumour agent ifosfamide and its metabolites. Ifosfamide is effective in a large number of malignant diseases. Its use, however, can be accompanied by haematological toxicity, neurotoxicity and nephrotoxicity. Since its development in the middle of the 1960s, most of the extensive metabolism of ifosfamide has been elucidated. Identification of specific isoenzymes responsible for ifosfamide metabolism may lead to an improved efficacy/toxicity ratio by modulation of the metabolic pathways. Whether ifosfamide is specifically transported by erythrocytes and which activated ifosfamide metabolites play a key role in this transport is currently being debated. In most clinical pharmacokinetic studies, the phenomenon of autoinduction has been observed, but the mechanism is not completely understood. Assessment of the pharmacokinetics of ifosfamide and metabolites has long been impaired by the lack of reliable bioanalytical assays. The recent development of improved bioanalytical assays has changed this dramatically, allowing extensive pharmacokinetic assessment, identifying key issues such as population differences in pharmacokinetic parameters, differences in elimination dependent upon route and schedule of administration, implications of the chirality of the drug and interpatient pharmacokinetic variability. The mechanisms of action of cytotoxicity, neurotoxicity, urotoxicity and nephrotoxicity have been pivotal issues in the assessment of the pharmacodynamics of ifosfamide. Correlations between the new insights into ifosfamide metabolism, pharmacokinetics and pharmacodynamics will rationalise the further development of therapeutic drug monitoring and dose individualisation of ifosfamide treatment.

Stereoselective pharmacokinetics of ifosfamide in male and female rats

The stereoselective pharmacokinetics of ifosfamide (IF) were investigated in male and female Sprague-Dawley rats. Following intravenous administration of IF deuterium-labeled pseudoracemates into rats at 40 mg/kg, IF enantiomers and their metabolites, 4-hydroxyIF (HOIF), N2-dechloroethylIF (N2D), N3-dechloroethylIF (N3D), and isophosphoramide mustard (IPM) were quantitated in plasma and urine using gas chromatographic-mass spectrometry techniques with appropriately deuterium-labeled analogs as the internal standards. In addition, the intrinsic clearances of IF isomers in rat liver microsomes were estimated by the in vitro metabolism study. Following drug administration in male rats, (R)-IF exhibited a lower area under the curve value and a shorter half-life of 34.2 minutes than (S)-IF, which gave a half-life of 41.8 minutes. In female rats, the half-lives of (R)- and (S)-IF were found to be 62.1 and 75.1 minutes, respectively, significantly longer than those in male rats. No change in volume of distribution or renal clearance for IF enantiomers in all rats was observed, and the protein binding value was low, with no enantioselectivity. Both in vitro and in vivo studies showed that metabolism of (R)-IF proceeded in favor of the 4-hydroxylation pathway, whereas (S)-IF preferentially underwent N2- and N3-dechloroethylation. The observed stereoselectivity and gender difference in pharmacokinetics of IF in the rat are mainly attributed to its stereoselective metabolism.

Metabolism and pharmacokinetics of oxazaphosphorines

The 2 most commonly used oxazaphosphorines are cyclophosphamide and ifosfamide, although other bifunctional mustard analogues continue to be investigated. The pharmacology of these agents is determined by their metabolism, since the parent drug is relatively inactive. For cyclophosphamide, elimination of the parent compound is by activation to the 4-hydroxy metabolite, although other minor pathways of inactivation also play a role. Ifosfamide is inactivated to a greater degree by dechloroethylation reactions. More robust assay methods for the 4-hydroxy metabolites may reveal more about the clinical pharmacology of these drugs, but at present the best pharmacodynamic data indicate an inverse relationship between plasma concentration of parent drug and either toxicity or antitumour effect. The metabolism of cyclophosphamide is of particular relevance in the application of high dose chemotherapy. The activation pathway of metabolism is saturable, such that at higher doses (greater than 2 to 4 g/m2) a greater proportion of the drug is eliminated as inactive metabolites. However, both cyclophosphamide and ifosfamide also act to induce their own metabolism. Since most high dose regimens require a continuous infusion or divided doses over several days, saturation of metabolism may be compensated for, in part, by auto-induction. Although a quantitative distinction may be made between the cytochrome P450 isoforms responsible for the activating 4-hydroxylation reaction and those which mediate the dechloroethylation reactions, selective induction of the activation pathway, or inhibition of the inactivating pathway, has not been demonstrated clinically. Mathematical models to describe and predict the relative contributions of saturation and autoinduction to the net activation of cyclophosphamide have been developed. However, these require careful validation and may not be applicable outside the exact regimen in which they were derived. A further complication is the chiral nature of these 2 drugs, with some suggestion that one enantiomer may have a favourable profile of metabolism over the other. That the oxazaphosphorines continue to be the subject of intensive investigation over 30 years after their introduction into clinical practice is partly because of their antitumour activity. Further advances in analytical and molecular pharmacological techniques may further optimise their use and allow rational design of more selective analogues.

High-performance liquid chromatographic determination of the enantiomers of cyclophosphamide in serum

A high-performance liquid chromatographic (HPLC) achiral-chiral coupled assay to measure the serum concentration of the enantiomers of cyclophosphamide is described. The R- and S-enantiomers of cyclophosphamide were quantified using a 5-cm-long C1 Spherisorb 5-microns column, with switching of the eluent containing racemic cyclophosphamide onto a 10-cm-long alpha 1 acid glycoprotein column. The limit of determination was 1.25 mg l-1 for each enantiomer and the ratio of the enantiomers over the range 2.5 to 100 mg l-1 was I. Serum enantiomer concentrations in blood samples taken from patients receiving 0.30 to 0.75 gm-2 of intravenous racemic cyclophosphamide could be measured at least three half-lives post dose. In six patients no significant difference in the clearance of R- and S-cyclophosphamide was found.

Stereoselective pharmacokinetics of ifosfamide and its 2- and 3-N-dechloroethylated metabolites in female cancer patients

The pharmacokinetics of the R and S enantiomers of ifosfamide (IFF) and of its 2- and 3-N-dechloroethylated metabolites (2-DCE-IFF and 3-DCE-IFF) were investigated in 14 cancer patients treated with a 3-h infusion of (R,S)-IFF (3 g/m2) with mesna uroprotection. An enantioselective gas chromatographic-mass spectrometric (GC-MS) assay was used to determine the concentrations in plasma and urine. The AUCs of (R)-IFF were significantly larger than those of (S)-IFF (2480 +/- 200 vs 1960 +/- 150 microM.h). The terminal half-lives (7.57 +/- 0.99 h) and mean residence times (11.17 +/- 1.10 h) of (R)-IFF were significantly longer than those of (S)-IFF, 6.03 +/- 0.82 h and 9.37 +/- 0.88 h, respectively. The mean volume of distribution at steady rate of (R)-IFF (25.68 +/- 0.80 l/m2) was slightly smaller than that of (S)-IFF (27.35 +/- 0.89 l/m2). While the renal clearances of (R)-IFF and (S)-IFF were similar, the nonrenal clearance was significantly lower for (R)-IFF (30.20 +/- 2.70 vs 41.40 +/- 3.55 ml/m2 per min) as was total clearance (41.52 +/- 2.90 vs 52.37 +/- 3.75 ml/m(2) per min). The AUC values for all of the DCE metabolites from (S)-IFF were significantly greater than those from (R)-IFF with 47% of the measured AUC accounted for by DCE from (S)-IFF compared to only 20% for (R)-IFF. Therefore, the enantioselective difference in IFF elimination can be partially explained by differences in N-dechloroethylation.